The present invention relates to a composition containing isoprene homopolymer or a butadiene-isoprene copolymer, and more particularly to a rubber composition containing as a rubber component an isoprene polymer having a glass transition temperature higher than a specific temperature.
Hitherto, many kinds of conjugated diene polymers are known, and in particular, isoprene polymers and butadiene polymers synthesized by using the so-called Ziegler catalyst and butadiene polymers synthesized by using an organolithium catalyst have been manufactured on an industrial scale. These polymers are characterized by their modes of microstructure, and it is known that they are polymers having a high cis content or a high cis and trans content. It is also known that natural rubber is an isoprene polymer having a high cis content. These natural and synthetic rubbers have been employed for various purposes such as tires.
In recent years, safety in running and low fuel consumption of automobiles have been increasingly demanded because of development of a superhighway system and a rise of energy cost. Consequently, there has been desired the improvement for the characteristics of automobile tires, particularly the force of gripping a wet road and the rolling resistance. Although the characteristics of tires vary depending on the structure of tire and the characteristics of raw materials of tire, in particular the wet road gripping force and the dynamic characteristics such as energy losses due to dynamic heat generation are greatly affected by the characteristics of a rubber material.
The above-mentioned isoprene and butadiene polymers having a high cis content which are presently used as a rubber component of compositions for tires, have a feature that dynamic heat generation is small. However, they have a drawback that the braking characteristics which are another important performance required in tires are bad, and in particular are remarkably lacking in the wet skid resistance which indicates the wet road gripping force. In order to cover up the drawback, there has been adopted a manner using a rubber blend. For instance, since cis-polybutadiene rubber is superior in the dynamic heat generation characteristic to isoprene rubber, and since a styrene-butadiene rubber having a high glass transition temperature has a good wet skid resistance though being large in dynamic heat generation, there has been employed as a rubber component of a composition for tires a blend of the cis-polybutadiene rubber and the styrene-butadiene rubber in a suitable proportion, into which natural rubber or a cis-polyisoprene rubber may be further incorporated, as occasion demands. However, this manner increases the dynamic heat generation, thus resulting in increase of the rolling resistance of tires, though the wet skid resistance is improved to some extent. There has not been provided a rubber composition which provides a rubber having a wet skid resistance as high as the styrene-butadiene rubber and a dynamic heat generation characteristic as low as the cis-polybutadiene rubber. It is the present condition in this industry that unsatisfactory rubber compositions are used, and rubber compositions showing a high wet skid resistance and a small dynamic heat generation characteristic have been strongly desired.